Middle English iron, iren, yron, from Old English iren, variant (with rhotacism of -s-) of isen, later form of isern, isærn "the metal iron; an iron weapon or instrument," from Proto-Germanic *isarnan (source also of Old Saxon isarn, Old Frisian isern, Old Norse isarn, Middle Dutch iser, Old High German isarn, German Eisen).

This probably is an early borrowing of Celtic *isarnon (compare Old Irish iarn, Welsh haiarn), which Watkins suggests is from PIE *is-(e)ro- "powerful, holy," from PIE *eis "strong" (source also of Sanskrit isirah "vigorous, strong," Greek ieros "strong"), on the notion of "holy metal" or "strong metal" (in contrast to softer bronze).

It was both an adjective and a noun in Old English, but in form it is an adjective. The alternative isen survived into early Middle English as izen. In southern England the Middle English word tended to be ire, yre, with loss of -n, perhaps regarded as an inflection; in the north and Scotland, however, the word tended to be contracted to irn, yrn, still detectable in dialect.

Right so as whil that Iren is hoot men sholden smyte. [Chaucer, c. 1386]

Chemical symbol Fe is from the Latin word for the metal, ferrum (see ferro-). Meaning "metal device used to press or smooth clothes" is from 1610s. Meaning "golf club with an iron head, 1842. To have (too) many irons in the fire "to be doing too much at once" is from 1540s. Iron lung "artificial respiration tank" is from 1932. The iron crown was that of the ancient kings of Lombardy, with a thin band of iron in the gold, said to have been forged from a nail of Christ's Cross. Iron horse "railroad locomotive" is from an 1839 poem. Iron maiden, instrument of torture, is from 1837 (probably translating German eiserne jungfrau). The unidentified French political prisoner known as the man in the iron mask died in the Bastille in 1703. In British history, Wellington was called the Iron Duke by 1832.